U.S. patent number 6,909,911 [Application Number 10/218,215] was granted by the patent office on 2005-06-21 for wireless terminal.
This patent grant is currently assigned to Koninklijke Philips Electronics N.V.. Invention is credited to Kevin R. Boyle, Peter J. Massey.
United States Patent |
6,909,911 |
Boyle , et al. |
June 21, 2005 |
Wireless terminal
Abstract
A capacitively back-coupled wireless terminal (10) comprises a
PCB (22) having a ground conductor (24) on one surface. A back
coupling capacitor (C) is carried by the ground conductor. An
elongate slot (30) is provided in the ground conductor to widen the
bandwidth. An electromagnetic shield (32) is provided on the side
of the PCB (22) which in use is facing a lossy dielectric in order
to reduce the SAR. The shield is disposed adjacent to, but spaced
from, the slot (30) in the ground conductor and the back coupling
capacitor. For narrowband operation the slot may be omitted and/or
the spacing between the PCB (22) and the shield (32) may be
reduced.
Inventors: |
Boyle; Kevin R. (Horsham,
GB), Massey; Peter J. (Horley, GB) |
Assignee: |
Koninklijke Philips Electronics
N.V. (Eindhoven, NL)
|
Family
ID: |
9922088 |
Appl.
No.: |
10/218,215 |
Filed: |
August 13, 2002 |
Foreign Application Priority Data
|
|
|
|
|
Sep 13, 2001 [GB] |
|
|
0122226 |
|
Current U.S.
Class: |
455/575.5;
343/702; 343/841; 455/300; 455/301 |
Current CPC
Class: |
H01Q
1/243 (20130101); H01Q 1/245 (20130101); H05K
1/0218 (20130101); H05K 1/0231 (20130101) |
Current International
Class: |
H01Q
1/24 (20060101); H05K 1/02 (20060101); H04M
001/00 (); H04B 001/10 (); H01Q 001/24 (); H04Q
001/52 () |
Field of
Search: |
;455/575.5,300,301,575.7,90.1,90.3,117
;343/841,851,702,700MS,846,848,824 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0986130 |
|
Mar 2000 |
|
EP |
|
1018779 |
|
Jul 2000 |
|
EP |
|
1132998 |
|
Sep 2001 |
|
EP |
|
WO0213306 |
|
Feb 2002 |
|
WO |
|
WO0237600 |
|
May 2002 |
|
WO |
|
WO02063712 |
|
Aug 2002 |
|
WO |
|
WO02065582 |
|
Aug 2002 |
|
WO |
|
Other References
Patent Abstracts of Japan, Maeda Takeyasu: "Radio Communication
Device" Publication No. 09027711, Jan. 28, 1997, Application No.
07176000, Jul. 12, 1995. .
Patent Abstracts of Japan, Ito Hirochika: "Antenna System And
Portable Radio Equipment" Publication No. 2000101333, Apr. 7, 2000,
Application No. 10266478, Sep. 21, 1998..
|
Primary Examiner: Nguyen; Duc M.
Attorney, Agent or Firm: Thorne; Gregory L.
Claims
What is claimed is:
1. A wireless terminal comprising a printed circuit board having a
ground conductor, a back-coupling capacitor mounted on the printed
circuit board and including a plate electrode spaced from a surface
of the printed circuit board, and a radiation blocking shield
spaced from a side of the printed circuit board which in use faces
a lossy side of the terminal.
2. A terminal as claimed in claim 1, characterised in that the
back-coupling capacitor is a parallel plate capacitor and in that a
portion of the ground conductor comprises a second plate
electrode.
3. A terminal as claimed in claim 1, characterised in that an
elongate slot is provided in the ground conductor and in that the
shield is coextensive with at least the entire length of the
slot.
4. A terminal as claimed in claim 3, characterised in that the
printed circuit board is elongate and the slot is parallel to the
major axis of the printed circuit board.
5. A terminal as claimed in claim 1, characterised in that an
elongate slot is provided in the ground plane, in that the
back-coupling capacitor is positioned adjacent the slot, and in
that the shield is mounted adjacent to, but spaced from, the slot
and the back-coupling capacitor and covers the slot.
6. A terminal as claimed in claim 1, characterised in that an
elongate slot is provided in the ground plane, in that the plate
electrode of the back-coupling capacitor is positioned to cover at
least part of the slot, and in that the shield is mounted adjacent
to, but spaced from, the slot and the back-coupling capacitor and
covers the slot.
7. A terminal as claimed in claim 1, characterised by a loudspeaker
and in that the shield is juxtaposed behind the loudspeaker to
prevent radiation propagating towards the loudspeaker.
Description
The present invention relates to a wireless terminal having
particular, but not exclusive, application as a cellular telephone
or low power remote control terminal. More particularly the present
invention relates to a wireless terminal having a planar
antenna.
Conventionally, planar inverted F antennas (PIFA) or similar are
used to provide low SAR (Specific Absorption Rate) performance on
handheld terminals such as mobile phones. PIFAs must have a
relatively large distance, typically 8 mm, between their top plate
and the supporting PCB in order to having sufficient bandwidth for
GSM (900 MHz) and DCS (1800 MHz) operation. This limits the
thickness of say the mobile phone.
International Patent Application WO 02/13306 (Applicant's reference
PHGB 010056WO) discloses a method of widening the bandwidth of a
radio communication terminal which uses the handset as a radiator.
More specifically in place of an antenna, a physically very small,
back coupling capacitor designed to have a large capacitance for
maximum coupling and minimum reactance is used. The residual
reactance of the back coupling capacitor can be tuned out with a
simple matching circuit. With such an arrangement, the bandwidth
can be greater than with a conventional antenna and handset
combination because the handset acts as a low Q radiator.
The bandwidth can be improved by redesigning the handset case to
increase the resistance in the return loss S.sub.11 after matching.
This was simulated using the High Frequency Structure Simulator
(HFSS) available from Ansoft Corporation. Experiments have shown
that the length of the handset could be optimised to give a wide
bandwidth centred on a particular frequency. A transversely
extending slot or longitudinally extending slot are means whereby
for a fixed length handset, electrical shortening or lengthening
can be achieved. Examples of longitudinally extending slots are
illustrated in FIGS. 11, 14 and 17 of the cited specification.
It is well known that in simulating the absorption of radiation,
the human body resembles a lossy dielectric which absorbs
electrical radiation. The local maxima of the body loss is measured
by the SAR (Specific Absorption Rate) performance. Losses of
radiated energy to the body represent a waste of energy which will
needlessly reduce battery life prematurely without enhancing the
overall performance of the terminal,
An object of the present invention is to improve the SAR of a
capacitively back-coupled handset.
According to the present invention there is provided a wireless
terminal comprising a printed circuit board having a ground
conductor, a back-coupling capacitor mounted on the printed circuit
board and including a plate electrode spaced from a surface of the
printed circuit board, and a radiation blocking shield spaced from
a side of the printed circuit board which in use faces a lossy side
of the terminal.
The advantages of providing a shield are that the SAR is improved
and that the shield can be placed relatively close to the printed
circuit board in the handset which will enable the terminal, for
example mobile telephone handset, to be slimmer and thereby more,
attractive to users.
An elongate slot may be provided in the ground conductor which will
enable the operating frequency bandwidth of the terminal to be
widened without the terminal having to be made thicker.
The present invention will now be described, by way of example,
with reference to the accompanying drawings, wherein:
FIG. 1 is a diagrammatic perspective view from the back of an
embodiment of a portable wireless terminal made in accordance with
the present invention,
FIG. 2 is a diagrammatic perspective view from the front of an
embodiment of a portable wireless terminal made in accordance with
the present invention,
FIG. 3 is graph of simulated return loss S.sub.11 in dB against
frequency f in GHz for the portable wireless terminal shown in
FIGS. 1 and 2,
FIG. 4 is a Smith chart showing the simulated impedance of the
portable wireless terminal shown in FIGS. 1 and 2 over the
frequency range 800.0 MHz to 3.0 GHz,
FIGS. 5 and 6 are graphs of real and imaginary parts of the antenna
input impedance in ohms against frequency f in GHz, respectively,
for the portable wireless terminal shown in FIGS. 1 and 2 having
the shield in place,
FIG. 7 is a sketch showing a portion of the printed circuit board
with the plate of the capacitor covering a portion of the slot,
FIG. 8 is a sketch showing a portion of the printed circuit board
having the capacitor but no slot,
FIG. 9 is a sketch showing a portion of the printed circuit board
having the slot extending transversely of the printed circuit
board, and
FIG. 10 is a sketch showing the capacitor mounted in the space
between the printed circuit board and the shield.
In the drawings the same reference numerals have been used to
indicate corresponding features.
The embodiment of the wireless terminal shown in FIGS. 1 and 2
comprises a capacitively back-coupled handset 10 suitable for use
as a cellular or cordless telephone. The handset 10 has an external
plastics housing 12, shown in broken lines, on the front side of
which are provided an aperture 14 for access to a microphone (not
shown), apertures 16 for access to a loudspeaker (not shown), a LCD
panel 18 and a keypad 20.
Within the housing 12 is mounted an elongate printed circuit board
22 carrying on at least one surface electronic components (not
shown) required for the control of the handset 10 and for the
associated processing of transmitted and received speech and data
signals. At least a portion of the other surface of the circuit
board 22 facing the back of the handset is metallised to provide a
ground plane 24.
The illustrated handset does not have a dedicated, separate antenna
but has a physically small back-coupling capacitor C designed to
have a large capacitance for maximum coupling and minimum
reactance. The residual reactance of the back-coupling capacitor
can be tuned out with a simple matching circuit, for example a
series inductor followed by a shunt inductor. By correct design of
the handset 10, the bandwidth can be much greater than with a
conventional antenna and handset combination, because the handset
acts as a low Q radiating element (simulations show a typical Q is
around 1), whereas conventional antennas typically have a Q of
around 50.
The capacitor C comprises a parallel plate capacitor formed by a
first plate consisting of a 10 mm.times.10 mm plate 26 mounted on
an insulated support 28 above the corner of the ground plane 24
which constitutes a second plate of the capacitor. The first plate
26 of the capacitor is fed by way of a transmission line (not
shown) which extends through the support 28. The resulting
capacitance is of the order of 0.5 pF, representing a compromise
between capacitance (which would be increased by reducing the
separation of the plate 26 and the ground plane 24) and the
coupling effectiveness (which depends on the separation of the
plate 26 and the ground plane).
In order to increase the bandwidth of the handset 10, a
longitudinally extending slot 30 is provided in the circuit board
22. The slot 30 is parallel to the length, or major axis, of the
circuit board. The dimensions of the slot 30 are chosen having
regard to the desired bandwidth of the handset 10 and for combined
GSM and DCS operation, the dimensions may typically be 3 mm wide
and 29.5 mm long, the circuit board typically being 100 mm.times.40
mm.times.1 mm.
An electrically conductive shield 32 is mounted on, and connected
electrically to, the circuit board 22 so as to extend parallel to,
but spaced from, the portion of the circuit board in which the slot
30 and the capacitor C are provided. The spacing between the shield
32 and the circuit board 22 is of the order of 4 mm. The depth of
the spacing is dependent on factors such as bandwidth. Thus if the
spacing is smaller than 4 mm, the bandwidth is narrower. As a
consequence a phone for use on a single standard as opposed to two
or more standards may be slimmer.
The shield 32 need not extend across the entire width of the
circuit board 22 but it should cover the capacitor C and the slot
30. The shield 32 may be mounted in any convenient way such as with
screws and spacers or with adhesives.
Simulations of a handset having a shield 32 have shown a
significant reduction in the magnetic field strength on the side
containing the loudspeaker earpiece, that is, the apertures 16.
This in turn leads to a lower SAR and less power loss to the user
of the handset. The overall thickness of the handset can be thinner
compared to a handset not having a shield.
FIG. 3 illustrates the return loss S.sub.11 using the High
Frequency Structure Simulator (HFSS), available from Ansoft
Corporation, with the results shown between 0.5 and 3.0 GHz. The
matching network comprised a series inductor of 4 nH followed by a
shunt inductor of 4 nH. The 6 dB bandwidth between the frequencies
m1 and m2, namely 1.823 and 2.065 Ghz, is approximately 200 MHz and
the 3 db bandwidth is significantly wider. Also due to the shape of
the S.sub.11 response with frequency, the bandwidth can be widened
with a parallel LC tuned circuit (with an easily realisable, small
inductance value. Such a circuit also has a useful filtering
function.
The Smith chart shown in FIG. 4 relates to a frequency band of
800.0 MHz to 3.00 GHz and the 6 dB bandwidth is shown between the
frequencies m1 and m2, namely 1.823 and 2.065 GHz.
FIGS. 5 and 6 are graphs showing the real and imaginary parts of
the antenna input impedance plotted against frequency when the
shield is present. These graphs confirm what is shown in the Smith
chart.
SAR simulations of the handset embodiment shown in FIGS. 1 and 2
were performed at 1800 MHz with the handset in close proximity to a
simple flat phantom, orientated in the same plane as the PCB 22
(i.e. parallel to it). The dielectric constant and conductivity of
the phantom were respectively .di-elect cons..sub.r =40 and
.sigma.=1.4 S/m. Three sets of results were generated, the first
without the shield 32 and with the circuit board 22 spaced from the
phantom by 5 mm, the second with the shield 32 present and touching
the phantom, and the third with the shield present and spaced from
the phantom by 1 mm. The following results were obtained for SAR in
W/kg:
W/kg: Accepted SAR at 1 W accepted power Antenna Power (W) Peak 1
gram 10 gram Unshielded 0.280 73.4 37.2 18.2 Shielded, touching
0.284 22.8 8.56 5.03 Shielded, 1 mm spacing 0.274 19.1 11.5
7.03
These results demonstrate that the shield 32 provides a
considerable reduction in SAR, typically reducing it to around 30%
of the value without a shield. This confirms the reduction in SAR
that would be expected from FIGS. 3 to 6.
FIG. 7 is a variant of the embodiment shown in FIGS. 1 and 2 in
that the plate 26 of the capacitor C overlaps the slot 30. Also for
the sake of illustration the shield, shown in broken lines, does
not extend over the entire width of the printed circuit board 32.
The width of the shield 32 is independent of the width of the slot
30 and/or the size of the plate 26, as long as the plate 26 is
covered.
The embodiment shown in FIG. 8 does not have a slot 30. However a
shield 32 is provided and covers the plate 26 of the capacitor.
Such an embodiment is suited to narrow bandwidth operation, for
example in a terminal operating in accordance with a single
standard, such as GSM or DCS.
Although in the embodiments described with reference to FIGS. 1, 2
and 7 the slot 30 in the ground plane has been shown extending
longitudinally of the printed circuit board 22, it may extend in
other directions, such as transversely of the elongate printed
circuit board subject to the orientation of the shield 32 being
changed accordingly. This is illustrated in FIG. 9 of the
accompanying drawings.
Referring to FIG. 10, this illustrates that the capacitor C can be
located on the side of the printed circuit board 22 facing the
shield 32.
In all the embodiments of the invention, the dielectric of the
capacitor C can be of any suitable type. However it is necessary
that one of the electrodes is a plate.
In the present specification and claims the word "a" or "an"
preceding an element does not exclude the presence of a plurality
of such elements. Further, the word "comprising" does not exclude
the presence of other elements or steps than those listed.
From reading the present disclosure, other modifications will be
apparent to persons skilled in the art. Such modifications may
involve other features which are already known in the design,
manufacture and use of capacitively back-coupled wireless terminals
and component parts therefor and which may be used instead of or in
addition to features already described herein.
* * * * *